mark IX

My Feedback: (9)

What is the relatonship between prop pitch speed and model level flight speed?

Thanx, Mark

ahicks

My Feedback: (2)

Wowee....

The short story is you need to balance the prop diameter and pitch to hold the engine at the top of it's power band.

The back story is if you want to go fast, you run more pitch with less diameter to get your balance. If you're looking for quicker acceleration and better vertical performance, or possibly easier speed control on your down lines (more drag), you would go with less pitch and more diameter.

mark IX

My Feedback: (9)

So there is no cook book equation that you can use to calculate level flight speed (LFS) from pitch speed (PS), correct? Having said that, I here that LFS is approx 80% of PS is that accurate?

Thanx, Mark

speedracerntrixie

My Feedback: (29)

Mark, I think you have that backwards. Reason being that on average once in the air the engine unloads and the prop becomes more efficient the faster you go. In the end it is a matter of how clean the airplane is.

ahicks

My Feedback: (2)

Re: the 80% you're thinking of, there's a few variables that need to be considered. Like airframe drag? Are we talking a Stearman or a P-51?

There are numerous "cook books" and spreadsheets that have been done if want to look into it further. I did a Google search on "prop speed vs. rpm spreadsheet" and found quite a bit of info you might like looking through...

mark IX

My Feedback: (9)

The model I am referring to is 1/5 scale Spitfire Mk IX

ahicks

My Feedback: (2)

You're wondering what prop size everyone is using on that plane, or do you want to dig into a little theory and see what SHOULD work?

What engine are you using/going to use?

BMatthews

The pitch speed depends on the engine or motor RPM as well. Feet per minute is going to be calculated by

"inches of pitch"/12 x RPM =FPM

From there just use conversions from online or from the units conversion in the Windows desktop calculator to find MPH.

For a scale Spit 80% to 85% for the hook up factor is likely a good value. But not all props have the whole blade or even more than a portion of the blade at the proper geometric pitch angle. If they did they'd look very odd and not at all normal to our eyes. So that just put in another variable. You also need to know for sure what the inflight RPM will be. And that is a very tough one to find.

So all in all in the end anything you calculate will be an educated guess at best. But it'll suggest a given range of prop pitch for you to try out.

Clean models manage to fly at pitch speeds that are 100% or even a hair more. We're talking seriously clean and slippery though. Meanwhile WW1 biplanes and similar are more like 65% to 70% for the pull factor.

mark IX

My Feedback: (9)

I am converting my 1/5 Spitfire Mk IX 83" to electric. The total weight turns out to be around 23-24 lbs. My batteries are (2 in series) 6S 8000maH. I've settled on using the Turnigy CA80-80-10 motor (max 6500W) along with a 200A speed controller, turning a Zinger 24" either 14 or 16 pitch 4 blade prop. I am trying to dertermine which prop/pitch combo would be best for all around use. Here is some data from the E-calc software with a prop pitch of 14:

At 12S and a 24" -14, 4 Blade prop:
Motor W = 6398
Motor RPM = 5232
Max motor temp = 145F
Pitch Speed = 70mph
Approx Lvl Speed = TBD
Flight Time = 4.5 min

At 12S and a 22" -16, 4 Blade prop:
Motor W = 5800
Motor RPM = 5432
Max motor temp = 136F
Pitch Speed = 82mph
Approx Lvl Speed = TBD
Flight Time = 5.2 min

airraptor

My Feedback: (66)

BH sorry but your thought dont apply to every plane even old wwI planes.

i flew a WM T-34 with an engine that turned a 12x4 APC at 16,800-17,200 in the air and it was at 90-92 mph..

Mark your plane will fly much faster than those speed posed there. I would go with the prop that is near the max of the motor you bought on a watt meter.

drac1

My Feedback: (4)

rpm X pitch X 60 / 39600 = KPH

airraptor

My Feedback: (66)

pitch speed calculators are now where near accurate

drac1

My Feedback: (4)

Hmm. Whatever.

I speed checked my plane with a radar gun and it was 121 V 129. Close enough.

049flyer

My Feedback: (18)

This one is pretty rough but still useful:

RPM in thousands X pitch in inches = approximate speed in MPH

So an engine turning a 10x5 inch prop at 12,000 RPM would be trying to pull the plane through the air at 60 mph.

Works OK for glow planes and small electrics but haven't yet fiddled with it on large planes.

drac1

My Feedback: (4)

Just need to divide by 1000 as well.

If the formulae is applied using ground RPM, then the calculated speed will be less than in the air when the engine unloads. It could gain up to 1000 RPM.

It's virtually the same formulae as I posted above, but mine gives the speed in KPH.

I don't see why it wouldn't be OK for larger planes. The same calculations would be used irrespective of the size of the plane.

airraptor

My Feedback: (66)

The reason they dont work is they dont account for the air foil on the prop and each MFG of props measures pitch differently.

Also not ever prop unloads in the air.

pitch calculators are useless just like knowing how much thrust an engine makes on the ground

speedracerntrixie

My Feedback: (29)

The pitch calculators don't account for the prop effeincency or airfoil just as Airraptor states. It's only taking into consideration of the pitch. The airfoil adds to the thrust generated. It also does not take into account the drag coefficient of the airplane. Drac's calculation is reasonably close because it was performed on a relatively draggy airplane.

drac1

My Feedback: (4)

Nope.

speedracerntrixie

My Feedback: (29)

Please share what airplane this was performed on. My opinion of a draggy airplane may be different then yours.

airraptor

My Feedback: (66)

speed i shouldnt have even started in here lol these go on and on we know what works and he knows what works. opinions are like #&$ holes everyone has one lol

mark use a watt meter to find a decent prop for your plane. those you have mentioned will perform ok as the best will be a two blade prop but since you want a 4 blade one those look good.

mark IX

My Feedback: (9)

Yeah, looks like it is not an exact science. The best thing to do is start testing. I just wanted to make sure I was headed in the right direction. One thing is for sure, the LFS is not greater than the PS which is what one of the LFS calculators that i was using was showing me and I was hanging my hat on. That, I just recently learned, and is important to know. Also finding a prop that utilizes, as close as possible, the motor's rated watts is another takeaway from these convesations, too. So all of your comments are valuable.

What I don't want is a slow flyer and I'm at nearly at full throttle. Would rather have a faster model @ 1/2 throttle and have some reserve throttle if needed. Not to mention the expense of trial and error.
I think I will start with a 2 blade 24" prop to get my power system set up and get some baseline data and experiment from there.

Thanx, Mark

speedracerntrixie

My Feedback: (29)

Mark, one thing to keep in mind that all props are not created equal. Your Zingers are at the bottom of the heap IMO. For a good flight prop you may want to look at some other brands. If it were me I would start with an APC 24x12 E prop or maybe a Xoar 24x14.

mark IX

My Feedback: (9)

The reason I am choosing Zinger is because they make a relatively in expensive 4 blade counterpart the to the 2 blade versions. It will give me an apples to apples comparison for my inital testing since the airfoil is also the same. Once I get more confidence in this power system I plan on looking at some of the more expensive 4 blade brands.

Thanx, Mark

rgburrill

The only real truth about anything related to aircraft is "theoretical is an approximation at best".

BobbyMcGee

So, does anyone know of any good TV shows that are on tonight?